Re: Portmap - was Re: Does mountd/statd really need to listen on a privileged port??

[Olaf Kirch <olaf.kirch@oracle.com>]

On Friday 27 April 2007 16:01, Peter Staubach wrote:
> It seems to me that you are still missing the point of the lock.
> 
> The lock is not being used to protect the assignment of the address
> of clnt_raw_private to clp, it is being used to ensure that clp is not
> being used before clnt_raw_private is completely initialized.
> 
> The lock is also used to ensure that multiple copies of clnt_raw_private
> are not allocated.

Are we talking of the same piece of code? Can you explain to me how
the following, which is  a copy of Bull's TIRPC library, ensures this?

clntraw_create:

        struct clntraw_private *clp = clntraw_private;
[...]

        mutex_lock(&clntraw_lock);
        if (clp == NULL) {
                clp = (struct clntraw_private *)calloc(1, sizeof (*clp));
                if (clp == NULL) {
                        mutex_unlock(&clntraw_lock);
                        return NULL;
                }
[...]
                clntraw_private = clp;
        }

Assume 2 threads enter clntraw_create simultaneously,
and clntraw_private is NULL.
Both execute clp = clntraw_private, and both see a
value of  NULL. Thread A grabs the mutex, thread B blocks.

Thread A finds clp == NULL, allocates memory, puts the
pointer into clntraw_private, does clever stuff and releases the
mutex.

Thread B wakes up, acquires the mutex. It checks
clp, which is still NULL, and allocates another
private object.

I really do think I understand the code, and there is
a problem. The mutex protects the clntraw_private
pointer, so the only safe thing to do is access that
variable while holding the lock, *not* before.

Olaf
-- 
Olaf Kirch  |  --- o --- Nous sommes du soleil we love when we play
okir@lst.de |    / | \   sol.dhoop.naytheet.ah kin.ir.samse.qurax

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